1. Field of the Invention
The present invention relates to a method for molding and vulcanizing rubber. More specifically, the present invention relates to a method for vulcanizing a rubber compound concurrently with molding thereof utilizing high frequency or very high frequency dielectric heating.
2. Description of the Prior Art
Generally, natural rubber (NR) and compound rubber such as styrene butadiene rubber (SBR), isobutylene isoprene rubber (TIR), nitrile butadiene rubber (NBR), isoprene rubber (IR), chloroprene rubber (CR) and etylene propylene copolymer (EPDM) have been utilized widely in various fields because of their elasticity, flexibility and gas and/or liquid sealing properties. Typically, the elastic properties of rubber are highly increased by the vulcanization technique. For vulcanization of rubber, there are various methods in the field which require heat treatment of the rubber at 120 to 200.degree. C. for a predetermined duration. However, the heating is time consuming because the thermal conductivity of rubber is very low. Therefore, dielectric heating which maintains high energy efficiency by utilizing a material's exothermic properties has been applied to vulcanization.
Dielectric heating is based on dielectric loss caused by the movement of dielectric polarized molecules in an electric field, according to Van der Waals force therebetween. When utilizing this method, dielectrics are heated uniformly and rapidly from within, generating heat by themselves. As is well known in the art, watts per cm.sup.3 of material (Pv) generated per unit volume at a unit duration is defined by the following formula; EQU Pv=5/9f.epsilon.' tan .delta.E.sup.2 .times.10.sup.-12 (W/cm.sup.3)(1)
wherein f :frequency,
.epsilon.':constant, PA2 tan .delta.:dielectric dissipation factor PA2 E: electrode voltage or the magnitude of the field strength.
As shown in the above formula (1), Pv is directly proportional to the dielectric loss factor which is represented as a product of the dielectric constant and the dissipation factor, i.e., .epsilon.' tan .delta., the frequency f, and the square of field strength.
Generally, NR and rubber compounds as previously mentioned have low dielectric loss factor (.epsilon. tan .delta.) as shown in the following Table 1.
TABLE 1 ______________________________________ Dielectric loss factor of rubber .epsilon.' at 20.degree. C. tan.delta. in 1MHz .epsilon.' tan.delta. ______________________________________ NR 2.0 0.003 0.006 SBR 2.0 0.003 0.006 IIR 2.0 0.001 0.002 NBR 2.0 0.0005 0.001 IR 2.0 0.002 0.004 EPDM 2.0 0.0005 0.001 CR 2.0 0.03 0.09 ______________________________________
It is clear from the formula (1), that f or E must be high in order to increase the temperature for heating. However, generating sufficiently high field strength (E) causes frequent discharging in an apparatus and causes safety problems and also lacks operational simplicity.
Taking all the above into consideration, microwave dielectric heating at ultra high frequencies (UHF) has been applied for vulcanizing rubber.
Japanese Patent First Publication (Tokkai) No.54-152082 and Japanese Patent First Publication No.56-4925 disclose methods for vulcanizing rubber which is compounded of exothermic agents which utilize microwave dielectric heating at 2450 MHz; (a UHF frequency). As previously mentioned, rubber compounds, when vulcanized have a relatively low dielectric loss of .epsilon. tan .delta., so the frequency (2450 MHz) is determined for inducing exothermic reaction. Exothermic agents, such as carbon black are blended in the rubber in order to promote the exothermic reaction. Typical methods of microwave dielectric heating are used in both disclosures. Microwaves generated in a magnetron generator are irradiated on the rubber compound located in an applicator through an waveguide. Thus, the rubber maybe heated sufficiently for vulcanization by microwave irradiation.
However, in the prior constructions, the rubber cannot be molded easily in the applicator during irradiation. Therefore, vulcanizing and molding must be done separately, requiring two steps. This increases manufacturing cost and complexity as well as requiring additional time for molding.